1) Remember there is a difference between imperial ton, US ton, and tonne. Find a conversion calculator.
2) As above, it's the gross vehicle weight (vehicle plus the maximum permissible load). 2.5 ton is probably the weight that the vehicle can carry, not including the weight of the vehicle.
3) If it's pre-1960 then you can drive some vehicles outside of your weight class as long as they are unladen (not carrying anything).
That's as I understand it anyway. If in doubt get some professional advice or check the uk.gov website for the DVLA leaflets etc.